Well, reloading one bullet at a time is nice when, for example, you still have 3 bullets left in your rifle and you don't want to expose yourself too much to that pesky enemy sniper holed-up in a building while you wait for the nearby allied reinforcements to close-in, ready to assault the building; you wouldn't want to be stuck in the middle of a reload just as they ran towards the entrance, where your fire support might make a noticeable difference.
Or when your squad is waiting by a door which you must go through, but you only have 2 bullets left in your rifle. Popping in 3 individual bullet makes more sense than shooting your two remaining bullets in the air or something, I think.
Then again, I don't know how hard is was to actually deconstruct stripper clips to isolate individual bullets, so my opinion on the subject is merely interpretative of imaginary concepts.